xref: /freebsd/lib/libsys/cap_rights_limit.2 (revision d2893828)

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.\" under sponsorship from the FreeBSD Foundation.
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.Dd April 27, 2024
.Dt CAP_RIGHTS_LIMIT 2
.Os
.Sh NAME
.Nm cap_rights_limit
.Nd limit capability rights
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In sys/capsicum.h
.Ft int
.Fn cap_rights_limit "int fd" "const cap_rights_t *rights"
.Sh DESCRIPTION
When a file descriptor is created by a function such as
.Xr fhopen 2 ,
.Xr kqueue 2 ,
.Xr mq_open 2 ,
.Xr open 2 ,
.Xr pdfork 2 ,
.Xr pipe 2 ,
.Xr shm_open 2 ,
.Xr socket 2
or
.Xr socketpair 2 ,
it is assigned all capability rights; for
.Xr accept 2 ,
.Xr accept4 2
or
.Xr openat 2 ,
it inherits capability rights from the "parent" file descriptor.
Those rights can be reduced (but never expanded) by using the
.Fn cap_rights_limit
system call.
Once capability rights are reduced, operations on the file descriptor will be
limited to those permitted by
.Fa rights .
.Pp
The
.Fa rights
argument should be prepared using
.Xr cap_rights_init 3
family of functions.
.Pp
Capability rights assigned to a file descriptor can be obtained with the
.Xr cap_rights_get 3
function.
.Pp
The complete list of the capability rights can be found in the
.Xr rights 4
manual page.
.Sh RETURN VALUES
.Rv -std
.Sh EXAMPLES
The following example demonstrates how to limit file descriptor capability
rights to allow reading only.
.Bd -literal
cap_rights_t setrights;
char buf[1];
int fd;

fd = open("/tmp/foo", O_RDWR);
if (fd < 0)
	err(1, "open() failed");

if (cap_enter() < 0)
	err(1, "cap_enter() failed");

cap_rights_init(&setrights, CAP_READ);
if (cap_rights_limit(fd, &setrights) < 0)
	err(1, "cap_rights_limit() failed");

buf[0] = 'X';

if (write(fd, buf, sizeof(buf)) > 0)
	errx(1, "write() succeeded!");

if (read(fd, buf, sizeof(buf)) < 0)
	err(1, "read() failed");
.Ed
.Sh ERRORS
.Fn cap_rights_limit
succeeds unless:
.Bl -tag -width Er
.It Bq Er EBADF
The
.Fa fd
argument is not a valid active descriptor.
.It Bq Er EINVAL
An invalid right has been requested in
.Fa rights .
.It Bq Er ENOSYS
The running kernel was compiled without
.Cd "options CAPABILITY_MODE" .
.It Bq Er ENOTCAPABLE
The
.Fa rights
argument contains capability rights not present for the given file descriptor.
Capability rights list can only be reduced, never expanded.
.El
.Sh SEE ALSO
.Xr accept 2 ,
.Xr accept4 2 ,
.Xr cap_enter 2 ,
.Xr fhopen 2 ,
.Xr kqueue 2 ,
.Xr mq_open 2 ,
.Xr open 2 ,
.Xr openat 2 ,
.Xr pdfork 2 ,
.Xr pipe 2 ,
.Xr read 2 ,
.Xr shm_open 2 ,
.Xr socket 2 ,
.Xr socketpair 2 ,
.Xr write 2 ,
.Xr cap_rights_get 3 ,
.Xr cap_rights_init 3 ,
.Xr err 3 ,
.Xr capsicum 4 ,
.Xr rights 4
.Sh HISTORY
The
.Fn cap_rights_limit
function first appeared in
.Fx 8.3 .
Support for capabilities and capabilities mode was developed as part of the
.Tn TrustedBSD
Project.
.Sh AUTHORS
This function was created by
.An Pawel Jakub Dawidek Aq Mt pawel@dawidek.net
under sponsorship of the FreeBSD Foundation.